A primary step in the fabrication of semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of vapour precursors. One known technique for depositing a thin film on a substrate is chemical vapour deposition (CVD). In this technique, process gases are supplied to a process chamber housing the substrate and react to form a thin film over the surface of the substrate. Examples of gases supplied to the process chamber to form a thin film include, but are not restricted to:                Silane and ammonia for the formation of a silicon nitride film;        Silane, ammonia and nitrous oxide for the formation of a SiON film;        TEOS and one of oxygen and ozone for the formation of a silicon oxide film; and        Al(CH3)3 and water vapour for the formation of an aluminium oxide film.        
Plasma etching processes are typically also performed in the process chamber to etch circuit features. Etching gases are typically perfluorocompound gases such as CF4, C2F6, CHF3, NF3 and SF6.
The inside surface of the process chamber is also regularly cleaned to remove the unwanted deposition material from the chamber. One method of cleaning the chamber is to supply a perfluorocompound cleaning gas such as NF3 or C2F6 to react with the unwanted deposition material.
A process tool typically has a plurality of process chambers, each of which may be at respective different stage in a deposition, etching or cleaning process, and so the gas being exhaust from the chambers at any given time may have various different compositions. During these processes, there is typically a residual amount of the gas supplied to the process chamber contained in the gas exhaust from the process chamber. Gases such as silane, and ammonia are highly dangerous if exhausted to the atmosphere, and perfluoro-compounds are greenhouse gases. In view of this, before the exhaust gas is vented to the atmosphere abatement apparatus is often provided to treat the exhaust gas. The abatement apparatus converts the more hazardous or undesirable components of the exhaust gas into species that can be readily removed from the exhaust gas, for example by conventional scrubbing, and/or can be safely exhausted to the atmosphere.
Silane, ammonia and perfluorocompounds (PFCs) such as NF3 and C2F6 can be removed from the gas stream with high efficiency using a microwave plasma abatement device. An example of a microwave plasma reactor is described in UK Patent no. GB 2,273,027. In that device, a waveguide conveys microwave radiation from a microwave generator into a resonant cavity housing two electrodes in a closely opposed relationship. A gas to be treated flows into the cavity through a gas inlet, and passes between the electrodes; so that a microwave plasma is initiated and sustained between the two electrodes from the gas flowing between the electrodes. One of the electrodes has an axial hole to provide a gas outlet from the resonant cavity. Under the intensive conditions within the plasma, species within the gas stream are subjected to impact with energetic electrons causing dissociation into reactive species that can combine with oxygen or hydrogen to produce relatively stable by-products. For example, C2F6 can be converted into CO, CO2 and HF, which can be removed in a further treatment step, and SiH4 can be converted into SiO2.
A microwave plasma abatement device may therefore be connected to the exhaust from each process chamber of the tool to treat the gas streams exhaust from the process tool, each device having its own microwave generator. However, as the gas streams exhausted from the process chambers typically have a relatively high flow rate, the microwave power required to both initiate the plasma and maintain an acceptable destruction and removal efficiency (DRE) tends to be relatively high, typically between 3 and 6 kW. Therefore, the costs associated with the microwave plasma abatement device tend to be relatively high.
As an alternative to using microwave plasma abatement devices, a dc plasma torch abatement device may be used to treat each gas stream exhaust from the process tool. As is known, a dc plasma torch creates a plasma flame from an inert, ionisable gas such as argon conveyed between the electrodes of the torch. The gas stream and a suitable source of hydrogen and oxygen can be conveyed into the plasma flame to react to form the relatively stable by-products mentioned above. Plasma torch abatement devices are relatively cheap in comparison to microwave plasma abatement devices, and are relatively cheap to operate, for example as a plurality of torches may be operated from a single dc power supply. However, the abatement performance for cleaning gases such as fluorocarbons and NF3 tends to be relatively low in comparison to that of microwave plasma abatement devices.
It is an aim of at least the preferred embodiments of the present invention to provide a plasma reactor that can combine the relatively high destruction efficiency of a microwave plasma abatement device with the low cost associated with a plasma torch abatement device.